1. Field of the Invention
The present invention relates to a positive photoresist composition and a process for forming a contact hole. More particularly, it relates to a positive photoresist composition for forming a contact hole, which provides a contact hole pattern image having less dimples and being in exact accordance with a mask pattern in the formation of contact holes with the use of the phaseshift technique. Additionally, it relates to a process for forming such contact holes.
2. Description of the Related Art
In manufacturing processes for semiconductor devices such as an integrated circuit (IC) and large-scale integrated circuit (LSI), and for liquid-crystal devices such as a liquid-crystal display (LCD), a positive photoresist composition comprising an alkali-soluble resin and a compound containing a quinonediazide group as a material having satisfactorily high definition, sensitivity and etching resistance is practically used.
Recently, a photolithography (optical lithography) technique using a reducing-type projection alignment method has markedly improved sensitivity of such a photoresist. With an increasing integration of 4-M, 64-M and then 256-M integration, integrated circuits require elements having still finer or smaller sizes on the order of a wavelength of a ray (0.3 to 0.5 .mu.m) used to print an element pattern. Accordingly, in production processes of integrated circuits, conventional photolithography techniques using an i-line (365 nm) reaches its limitation upon improvement of definition, and hence there are demands for technologies to improve definition of photoresists.
As an example of such technologies, attempts have been made to change a source to any of shorter wavelength rays such as excimer laser beams from KrF, ArF or the like, or electron beams, X-rays or other rays. These techniques have not yet been widely employed because they require change of equipment, involving enormous increase in its cost. To this end, there are demands for techniques to enhance definition at a low cost without change of a source. As an illustrative example, there has been proposed a phaseshift method ("Technologic Innovation in USLI Lithography", pp. 34-40, issued by Science Forum Ltd., Nov. 19, 1994).
The phaseshift method involves a process for emphasizing contrast of a ray which transmits an aperture (a ray-transmittable portion) by processing a mask substrate for printing a circuit pattern on a photoresist film so as to allow phase of the ray to shift partly. A mask substrate to which the phaseshift technique is not applied is composed of apertures (ray-transmittable portions) that transmit a ray and masking portions which do not. As rays transmit via apertures and diffuse, such diffused rays overlap each other in case of a fine pattern, and hence fail to give separate patterns (that is, definition is deteriorated). The phaseshift technique provides partial phaseshift of rays, emphasizes contrast of the rays and hence improves definition.
The phaseshift method includes spatial frequency modulation type (Levenson type), edge reinforcement type, multi-phase transmittance control type and other types of techniques. Of these techniques, the edge reinforcement type technique and the multi-phase transmittance control type technique are favorable for the formation of contact hole patterns. Particularly, it is thought that the multi-phase transmittance control type technique using a halftone mask will become useful one.
Use of the halftone mask has been widely reported, and disclosed, for instance, in Japanese Patent Laid-Open Nos. 8-137089, 8-171196, 8-279452 and others.
Further, Japanese Patent Laid-Open No. 9-96904 discloses a positive photoresist composition for forming contact hole.
The halftone mask is a mask obtained by composing its masking portions of a translucent film that transmit rays in a transmittance of several percents, while such masking portions are generally composed of a chromium film which does not transmits rays. This construction allows a ray transmitted via an aperture and another ray transmitted via the masking portion composed of the translucent film to be in reversed or opposite phase to each other, and hence sharpens a light intensity distribution of a main pattern, since such reversed phase rays counteract each other. Thus, it gives a contact hole pattern having a high contrast and an improved definition.
Most of these halftone masks and other phaseshift masks are, however, disadvantageous in that they cause the formation of sub-patterns of ray, that is, so-called side lobes, in the periphery of a main pattern (Japanese Patent Laid-Open No. 8-222513). Though such side lobes have a weaker light intensity than in a main pattern, they invite, upon the formation of a fine pattern, the formation of concaves, namely, so-called dimples in other portions than the main pattern (i.e., photoresist-coated portions corresponding to the side lobes) after development. These dimples deteriorate characteristics or properties of the photoresist film and cause defects such as interconnection failure.